The Advanced User Guide for the Acorn Electron
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- 5.2 OSARGS Read/write open file’s attributes Return current filing system
- 5.3 OSBGET Get a single byte from an open file
- 5.4 OSBPUT Write a single byte to an open file
- 5.5 OSGBPB Read/write a group of bytes to/from an open file
- 5.6 OSFIND Open or close file for random access
- 5.7 OSFSC Miscellaneous filing system control
- 6 Operating System Vectors
- OS and filing system calls indirection vectors
- 6.1 The User Vector 200
Bit Meaning if set0 not readable by you 1 not writable by you 2 not executable by you 3 not deletable by you 4 not readable by others 5 not writable by others 6 not executable by others 7 not deletable by others The term you here means the originator of the call and the term others means other users of a network filing system. The action codes passed to OSFILE in the accumulator have the following effects: A=0 Save a section of memory as a named file using the information supplied in the parameter block. A=1 Re-write the catalogue information of an existing file using the information provided in the parameter block. i.e. load and execution addresses. A=2 Re-write the load address (only) of an existing file identified by the name passed in the parameter block. A=3 Re-write the execution address (only) of an existing file identified by the name passed in the parameter block. A=4 Re-write the file attributes (only) of an existing file identified by the name passed in the parameter block. A=5 Read the named file’s catalogue entry and return the file type in the accumulator. These are as follows: 0 returned in A Nothing found 1 returned in A File found 2 returned in A Directory found
Delete the named file. A=7 Create a file with a catalogue entry representing the parameter block information but instead of transferring any data pad with null characters. A=&FF Load the named file into memory. If the first byte of the execution address field of the parameter block is zero then load to the load address given in the parameter block. If the first byte of the execution address is non-zero then use the file’s own load address. During this call if an error occurs a BRK instruction will be executed which may be trapped if required. During this call interrupts may be enabled but the interrupt status is preserved. On exit: A contains the file type X and Y are preserved C, N, V and Z are undefined Information may be written to the parameter block addressed by X+Y. 5.2 OSARGS Read/write open file’s attributes Return current filing system Call address &FFDA Indirected through &214 This routine is used to manipulate files which are being used for random access. Files used in this way have to be opened using the OSFIND call. When data is being written to or read from the file OSBPUT, OSBGET and OSGBPB can be used but this call should be used to move the sequential pointer used by these calls when data is not transferred. This call is the only way of moving the sequential pointer backwards through a file. OSARGS may also be used to force an update of files onto the medium in use i.e. ensuring that the latest copy of the memory buffer is saved. A number of other functions are performed by this call as detailed below. Entry parameters: A contains a value determining the call’s actions X contains a zero page address of a parameter block Y contains the file handle (see OSFIND) or zero The parameter block in zero page should be in the user’s allocation of zero page. A block of four bytes is required, this will contain the value of the sequential file pointer for read operations or should be set up with a value prior to the call for a write operation. It should be noted that because filing systems should not be languages and so are not copied across to a second processor this parameter block will always exist in the I/O processor even when a Tube is active. If called from the second processor, the parameter block will be copied across into the I/O processor before the filing system is called. Interrupts may be enabled during a call but the interrupt status will be preserved. If Y=0 and A=0 then return the current filing system in A. value returned filing system 0 no current filing system
PTR#.
Write sequential file pointer. A=2 Read length of sequential file. This value is the same as that returned by EXT# in BASIC. A=3
A= &FF Update this file onto the filing system medium. On exit: A is preserved except on a call with A=0 and Y=0 X and Y are preserved
D=0
5.3 OSBGET Get a single byte from an open file Call address &FFD7 Indirected through &216 This routine returns the value of a byte read from a file opened for random access. The file should have been previously opened using OSFIND, The file handle required by this call will have been provided from this OSFIND call. Entry parameters: Y contains file handle A byte is read from that point in the file determined by the sequential file pointer. During each call of OSB GET the sequential file pointer is incremented by one. Thus successive OSBGET calls can be used to read bytes from the file sequentially. This pointer may be read or written using the OSARGS call thus enabling the use of random access. Interrupts may be enabled during a call but the interrupt status will be preserved. A is returned containing the value of the byte read. On exit: X and Y are preserved C= 1 if the end of file was reached i.e. invalid call , in which case A=&FE.
A contains the byte to be written to the file. Y contains the file handle.
A, X and Y are preserved C, N, V and Z are undefined 5.5 OSGBPB Read/write a group of bytes to/from an open file Call address &FFD1 Indirected through &21A This routine enables the transfer of a group of bytes to or from an open file. This routine is implemented particularly for filing systems which have a high time overhead associated with each data transfer e.g. the Econet. Entry parameters: A contains a value which determines the action performed X+Y contain a pointer to a parameter block in memory The parameter block should contain information in the following format: &00 file handle &01 - &04 address of data for transfer &05 - &08 number of bytes to transfer &09 - &0C sequential file pointer to be used The bytes in each parameter should be placed least significant byte first. The address should include a high order address (see OSBYTE &82) to indicate if the data is in an i/o processor or a second processor. The sequential file pointer passed in the parameter block will only replace the old value of this pointer when appropriate. The action codes passed to the routine will have the following effects: A=1
A=2 Write a group of bytes to the open file without using the sequential file pointer value given in the parameter block. The existing value of the pointer will mark the point in the file where these bytes are put and the pointer will then be incremented by the number of bytes written. A=3 Read a group of bytes from an open file. The sequential pointer given in the parameter block will indicate where the bytes should be read from within the file. The pointer will then be incremented by the number of bytes read. A=4 Read a group of bytes from an open file disregarding the sequential file pointer value given in the parameter block. The existing pointer value will be used and subsequently incremented by the number of bytes read. A=5 Return the title associated with the currently active medium and return boot/startup attribute, This information is written to the address pointed at by the X and Y registers. The format of the data is: &00 n, the length of the title string &01 - n+1 the title string, ASCII characters n+2 value indicating boot/start up options
A=6
&00 n, the length of the directory name &01 - n+1 directory name, ASCII string n+2 m, the length of the device identity n+3 - n+m+3 the device identity A=7 Read the currently selected library, and device, The data format is the same as that used for A=6. A=8 This call is used to read file names from the current directory. The parameter block should be set up so that the number of file names to transfer is placed in the ‘No. of bytes to transfer’ field, For the first call the ‘sequential file pointer’ field should be set to zero. The sequential file pointer is incremented each time this call is made so that it points to the next file name for transfer. The data is transferred to the specified address in the form of a list of file names. Each file name takes the form of an ASCII string preceded by a single byte value indicating the length of the string. The number of filenames in this list is determined by the value passed in the parameter block unless the end of the directory is reached. This call also returns a cycle number in the ‘file handle’ field of the parameter block. This cycle number represents the number of times the current catalogue has been written to. Exit conditions: A, X and Y are preserved N, V and Z are undefined C= 1 if the transfer could not be completed
Entry parameters (a) To open a file The accumulator contains a code indicating the type of access for which the file should be opened: A=&40 input only A=&80 output only (i.e. will attempt to delete file first) A=&C0 input and output
A returns file handle on opening otherwise preserved X and Y are preserved
Interrupts may be enabled during call, status preserved 5.7 OSFSC Miscellaneous filing system control No OS call address Indirected through &21E This vector contains an entry point into the current filing system which may be used to invoke a number of miscellaneous filing system functions. Because there is no direct call address this call can only be made from within an I/O processor and is not available for code running on a second processor. However many of the facilities are indirectly available via other OS calls which subsequently make calls through this vector. Entry parameters: The accumulator contains an action code determining which control function is performed. A=0 *OPT command The operating system makes this call in response to ‘*OPT’ being submitted to the command line interpreter or in response to OSBYTE &8B. X and Y contain the parameters passed with the ‘*OPT’ command. A= 1 Check for end of file (EOF) This call is made by the operating system in response to OSBYTE &7F. The call is entered with a file handle value in the X register. The X register should be returned containing the value &FF if an EOF condition exists, otherwise it should be returned containing zero. A=2 ‘*/’ command The filing system should attempt to *RUN the file whose name follows the ‘/’ character. The operating system command line interpreter will make this call in response to a command beginning ‘*/’. The X and Y registers contain the address of the file name string (not including the ‘*/’ characters). A=3 Unrecognised *command The operating system issues this call when an unrecognised command has been submitted to the command line interpreter. This call is made after the ‘unrecognised *command’ paged ROM service call has been made (see paged ROMs section 10.1). The command name string is addressed by the X and Y registers. Filing systems will respond to this call by attempting to *RUN the file having the command name. The idea behind this is to enable the implementation of command like utilities which are stored on the filing system media. However in the case of a filing system being unable to execute the file without delay the filing system should respond to this call with a ‘Bad Command’ message instead. A=4 *RUN attempted The operating system passes on the file name given with a *RUN command to the current filing system using this call. The X and Y registers contain the address of the file name string, The filing system should then load and execute the code in this file. A=5 *CAT attempted This call is made by the operating system in response to a *CAT command. The X and Y registers contain the address of the rest of the command string where any parameters required by the routine may be found. A=6 New filing system selected This call is issued when the current filing system is being changed. The deselected filing system should respond by closing any *SPOOL or *EXEC files using OSBYTE &77 and prepare itself for the handover. A=7 Return handle range This call may be made to determine the range of values allocated as file handles by the currently selected filing system. Below is a list of the handle ranges that have been allocated by Acorn. filing system handle range, inclusive Tape filing system 1 (&01) 2 (&02) *ROM filing system 3 (&03) 3 (&03) Teletext filing system 14 (&0E) 15 (&0F) Disc filing system 17 (&12) 21 (&15) Network filing system 32 (&20) 39 (&29) Winchester DFS 48 (&30) 57 (&39) reserved values 64 (&40) 71 (&49) Acacia RAM filing system 96 (&60) 101 (&65) IEEE filing system 240 (&F0) 255 (&FF)
A=8 OS *command about to be processed The operating system makes this call prior to executing a *command. Acorn DFS uses this call to implement the ‘*ENABLE’ protection mechanism. This call may also be used by filing systems to output extra messages e.g. ‘Compaction recommended’ when free space has become highly fragmented on a disc. On exit: Registers returned as described above Otherwise registers undefined Status flags undefined Interrupts may be enabled, status preserved
20 WRCHV=&20E 30 FOR opt%=0 TO 3 STEP3 40 P%=code% 50 [ 60 OPT opt% 70 .init LDA WRCHV \ A=lo byte of vector 80 STA ret_vec \ make a copy 90 LDA WRCHV+1 \ A=hi byte of vector 100 STA ret_vec+1 \ make a copy 110 LDX #intrcpt AND &FF \ X=lo byte of new routine 120 LDY #intrcpt DIV &100 \ Y=hi byte of new routine 130 SEI \ disable interrupts 140 STX WRCHV \ store new routine address 150 STY WRCHV+1 \ in WRCH Vector 160 CLI \ enable interrupts 170 RTS \ finished initialisation 180 .intrcpt CMP #ASC”£” \ trying to print a £ ? 190 BEQ pound \ if so branch 200 CMP #ASC”$” \ trying to print a $ ? 210 BEQ dollar \ if so branch 220 JMP (ret_vec) \ neither goto old routine 230 .pound LDA #ASC”$” \ replace £ with $ 240 JMP (ret_vec) \ goto old routine 250 .dollar LDA #ASC”£” \ replace $ with £ 260 JMP (ret_vec) \ goto old routine 270 .ret_vec EQUW 0 \ space for return vector 280 ]
290 NEXT 300 CALL init This program, although not very long, illustrates a few points regarding the way in which vectors should be intercepted. One of the most important aspects concerning the interception of calls through vectors is to make sure that the call is passed on to the previous owner of the vector. There are occasions when a routine is intended to be the sole replacement of a vector but as a rule it is good programming practice to copy the old vector contents to a returning vector. By returning via the old vector contents any number of intercepting routines can be daisy chained into the operating system call. While the initialising routine is changing the vector contents to point at the new routine it is wise to disable interrupts, It would obviously be quite catastrophic if the OSWRCH routine were to be called when the vector was only half changed. An interrupt handling routine is unlikely to use the WRCHV but there is no reason why it should not. The intention in this section has been to make programmers aware of the problems which may occur when intercepting these vectors. They have been implemented so that they may be used to insert extra code into some of the operating system routines and individuals should not be afraid of using them to this end. However, careful thought is required; take full account of the ramifications of altering the operating systems usual response to calls. If in doubt try out a routine. Play about with trivial examples such as the one given above. There is nothing to be lost and much to be learnt. OS and filing system calls indirection vectors The vector addresses associated with those operating system calls which are indirected are given in the detailed description of each call in chapter 2. The entry conditions with which the routine whose address is contained within these vectors will be unchanged from the initial OS call. Other page 2 vectors The other vectors reserved for containing the addresses of other operating system and miscellaneous routines are described below. These are: Name addr. description USERV &200 The user vector BRKV &202 The BRK vector IRQ1V &204 Primary interrupt vector IRQ2V &206 Unrecognised IRQ vector FSCV &21E File system control entry EVNTV &220 Event vector UPTV &222 User print routine NETV &224 Econet vector VDUV &226 Unrecognised VDU commands KEYV &228 Keyboard vector INSV &22A Insert into buffer vector REMV &22C Remove from buffer vector CNPV &22E Count/purge buffer vector IND1V &230 unused/reserved for future expansion IND2V &232 unused/reserved for future expansion IND3V &234 unused/reserved for future expansion 6.1 The User Vector &200 The user vector is called by the operating system in three circumstances: (a) When *CODE is passed to the command line interpreter The *CODE command takes two parameters which are placed in the X and Y registers. The user vector is then called with an accumulator value of zero. OSBYTE &88 may also be used to generate a *CODE command. (b) When *LINE is passed to the command line interpreter The *LINE command takes a line of text as a parameter. The user vector is entered with the X and Y registers containing the address of this text and A= 1. (c) When an OSWORD call &E0 to &FF has been made. The user vector is entered with the register values they were when the original OSWORD call was made. The default address stored in this vector points to a routine which generates an error with the message ‘Bad command’ and error number &FE. This vector is fully implemented on the BBC microcomputer and the Electron. On a Tube machine only the vector on the I/O processor is offered these calls. Listed below is a program which assembles a routine to intercept calls made to the user vector. It may be noticed that this routine does not offer the calls back to the original vector routine, this is because the default routine generates an error. There should only be one user vector handling routine active at any one time. 0 REM User vector handling routine 10 DIM code% &100 20 OSASCI=&FFE3 30 USERV=&200 40 FOR opt%=0 TO 3 STEP 3 50 P%=code% 60 [ 70 OPT opt% 80 .init LDX #userrt AND &FF \ X=lo byte of routine addr. 90 LDY #userrt DIV &100 \ Y=hi byte of routine addr. 100 SEI \ disable interrupts 110 STX USERV \ set up vector with addr. 120 STY USERV+1 130 CLI \ enable interrupts 140 RTS \ and return 150 .userrt CMP #1 \ compare contents of A with1 160 BCC code \ A<1 then must be *CODE 170 BNE osword \ now if A<>1 must be OSWORD 180 STX &70 \ *LINE routine 190 STY &71 \ store text address in page0 200 LDY #&FF \ set Y as loop counter 210 .loop INY \ beginning of loop Y=Y+1 220 LDA (&70),Y \ load first byte of string 230 JSR OSASCI \ print it 240 CMP #&D \ was character a cr? 250 BNE loop \ if not get the next char. 260 RTS \ if it was return 290 .code TXA \ A=X 300 JSR prntbt \ print value of X 310 JSR space \ print a space 320 TYA \ A=Y 330 JSR prntbt \ print value of Y 340 JMP new_ln \ print newline and return 350 .osword PHA \ save contents of A 360 LDX #&FF \ set X as loop counter 370 .loop1 INX \ beginning of loop, X=X+1 380 LDA string,X \ load character from string 390 JSR OSASCI \ print it 400 CMP #ASC”&” \ & char. is end of string 410 BNE loop1 \ loop if not end of string 420 PLA \ reload the value of A 430 JSR prntbt \ print it out in hex 440 JMP new_ln \ print cr and return
460 JMP OSASCI \ print space and return 470 .new_ln LDA #&D \ A=carriage return character 480 JMP OSASCI \ print cr and return 490 .string EQUS “OSWORD &“ \ string for OSWORD routine 499 \*** This routine prints hex number given in A 500 .prntbt PHA \ save copy of accumulator 510 LSR A 520 LSR A 530 LSR A 540 LSR A \ shift nibble hi to lo 550 JSR nibble \ print hi nibble hex digit 560 PLA \ reload accumulator 570 .nibble AND #&0F \ mask out high nibble 580 CMP #&0A \ digit or letter? 590 BCC number \ A<10 print number 600 ADC #&06 \ otherwise add 7 (C=1) 610 .number ADC #&30 \ add &30 to convert to ASCII 620 JMP OSASCI \ print character and return 630 ]
640 NEXT 650 CALL init Once assembled this routine will respond to *CODE by printing out the parameters passed with the command. A *LINE command will result in the parameter string being repeated on the screen and an OSWORD in the region &E0 to &FF will print out the number of the call. e.g.
01 02 >*LINE SOME TEXT SOME TEXT >A%=&E0:CALL &FFF1 OSWORD &E0 >
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